After the guns of World War I fell silent, the world’s nations convened in Geneva to outlaw for the first time an entire class of weapons. Barely 1 percent of the war’s battlefield deaths had come from toxic chemicals, yet these had evoked greater horror than the blast wounds, shrapnel and bullets that killed millions more.

Nearly a century later, images of another chemical-weapons attack are stirring some Western capitals into action, this time over the alleged gassing of thousands of Syrian civilians by their government. Now, as then, the toll of dead and injured is relatively small, roughly equal to the average number of fatalities in an ordinary week in Syria’s civil war.

The response has prompted the same question that arose during the debates of the early 1920s: Why are chemical weapons different?

The question is central to arguments being made this week by both proponents and opponents of a military strike to punish Syria over its alleged use of nerve gas in an Aug. 21 attack on rebel strongholds near eastern Damascus. President Obama has accused Syria of crossing a moral and legal red lineby using weapons that are outlawed and uniquely repugnant, and on Saturday he denounced the alleged attack as “an assault on our human dignity.”

But others question why the United States is compelled to respond to one type of killing when it took no military action to prevent the deaths of an estimated 100,000 Syrians by more conventional but often brutal methods.

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Even some who were convinced by the administration’s intelligence case say they question why a single atrocity ranks higher than so many others. The White House released an intelligence assessment and map Friday detailing why it holds Syrian officials responsible for the deaths of nearly 1,500 people, including more than 400 children.

“Even if the map is only half accurate, this was truly a heinous use of chemical weapons,” said Lawrence Wilkerson, a retired army colonel and former adviser to then-Secretary of State Colin Powell during the months before the 2003 U.S. invasion of Iraq. “That said, [North Korean leaders] Kim Il Sung and Kim Jong Il killed thousands if not millions more with starvation, yet we did nothing substantive. Is it worse to die of gas or hunger?”

Others argue that chemical weapons are indeed unique, and any use demands a firm response from civilized nations to deter future attacks.

“Chemical weapons are genuinely horrible, and they are indiscriminate,” said Jeffrey Lewis, an arms-control expert at the James Martin Center for Nonproliferation Studies. “They are a particularly cruel way to kill someone.”

It is true, Lewis said, that civilians will die in any conflict — often in excruciating ways. “But I’m on board with any effort to try to ban the most inhumane weapons, the things that kill on a mass scale,” he said.

Even before the first clouds of deadly chlorine gas swept over French lines in 1915, the use of poisons in warfare was widely seen as taboo. In 1863, in the middle of America’s bloodiest war, the U.S. War Department issued a decree banning poisoning in any form, including the use of poison-tipped bullets or the poisoning of food supplies. Eleven years later, a similar ban was approved, but never ratified, by 14 European countries.

Jonathan Tucker, the author of “War of Nerves,” a history of chemical weapons, argued that there’s something primal in the human revulsion to such weapons.

“This ‘chemical weapons taboo’ appears to have originated in the innate human aversion to poisonous substances,” Tucker writes, “as well as revulsion at the duplicitous use of poison by the weak to defeat the strong without a fair physical fight.”

Chemical toxins only became a true weapon of mass destruction during World War I, when European chemists were enlisted on both sides to help end the stalemate in the trenches. Chlorine, phosgene and mustard gas would eventually kill more than 90,000 soldiers and wound nearly a million others. While the death toll was relatively small — three times as many were killed in the Battle of the Somme in 1916 — gas attacks were feared because they frequently left victims in agony for days.

Public disgust over the use of such toxins helped drive support for the 1925 Geneva treaty banning the use of chemical weapons, as well as the more robust 1997 Chemical Weapons Convention that has been ratified by all but a handful of the world’s nations. Among the remaining holdouts are North Korea and Syria, both of which are said by U.S. officials to possess stockpiles of chemical munitions.

Since the 1925 treaty signing, only a handful of chemical attacks have been documented. Among the worst was a 1988 attack by Iraqi President Saddam Hussein against the Kurdish town of Halabja, in which sarin and other deadly gases killed between 3,000 and 5,000 civilians.

The Halabja deaths brought an international outcry but no military response. It was the last mass-casualty attack using chemical weapons until 2012, when U.S. officials say the Syrian regime launched the first in a series of strikes that culminated in the shelling east of Damascus on Aug. 21.

In his statement on Saturday, Obama evoked images of gassed Syrian children.

“What message will we send if a dictator can gas hundreds of children to death in plain sight and pay no price?” the president said. “What’s the purpose of the international system that we’ve built if a prohibition on the use of chemical weapons that has been agreed to by the governments of 98 percent of the world’s people and approved overwhelmingly by the Congress of the United States is not enforced?”

Some arms-control experts see merit in Obama’s insistence on accountability for those that violate such a widely shared principle on acceptable conduct in war. Daryl Kimball, executive director of the Washington-based Arms Control Association, said the century-old movement to eliminate chemical weapons lost ground when Saddam was permitted to gas his country’s Kurdish population with relative impunity.

“The muted response by the international community — including the United States — probably encouraged Saddam to continue using chemical weapons in that war, and also encouraged other countries in the region to develop their own,” Kimball said.

Greg Thielmann, a former State Department intelligence analyst who publicly questioned the George W. Bush administration’s case for invading Iraq in 2003, said the Syria evidence possesses a moral clarity that was missing 10 years ago. Back then, he said, a different White House was attempting to assess Iraq’s weapons capability and whether it might have nuclear weapons in the future. This time, the question is far simpler: Did Syria violate international law and a widely held “chemical weapons taboo”?